psusan: fix assertion failure in SFTP server.

Uppity's built-in SFTP server makes up its file handle identifiers
using random_read(). But when that server is reused in psusan, which
doesn't have the random number generator enabled, you get an assertion
failure.

sshserver.h

@@ -132,3 +132,6 @@ int platform_make_x11_server(Plug *plug, const char *progname, int mindisp,
                              Socket **sockets, Conf *conf);
 
 Conf *make_ssh_server_conf(void);
+
+/* Provided by Unix front end programs to uxsftpserver.c */
+void make_unix_sftp_filehandle_key(void *data, size_t size);